If by reliably you mean control him long enough to hit him or get a good head start. we do a agree.
If you mean getting someone stuck so that he can not escape when he actively try, like the pin you have you have with nikkio sankio pin (and some ikkio variations). I would say that you are not doing the standard pin.
That is pinning via the arm alone and that leaves him option to escape by the hips.

Even if you arch his spine, via the arm pin, and from the way you describe it we are using the same pin. He end up in a position no so different from "umpa", a bjjer or a wrestler can use that to get away.
But may be that is probably what you meant by uke being flexible.

You need at least a knee (or sitting) on the chest belly to prevent him to use his hips meaningfully and that position will help to keep the elbow down close to head an push the forearm outside. But we are quite far from the standard pin.

phil

One Ringeck to bring them all and in darkness bind them,
In the Land of Windsor where phlip phlop live.